Method for producing orthocarbonic acid trialkyl esters

ABSTRACT

A process is provided for the preparation of trialkyl orthocarboxylates by the electrochemical oxidation of alpha, beta-diketones or alpha, beta-hydroxyketones, the keto group being present in the form of a ketal group derived from C 1 - to C 4 -alkylalcohols and the hydroxyl group optionally being present in the form of an ether group derived from C 1 - to C 4 -alkylalcohols (ketals K), in the presence of C 1 - to C 4 -alcohols (alcohols A), the molar ratio of the ketals K to the alcohols A in the electrolyte being 0.2:1 to 10:1.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 National Stage Application of PCT/EP01/10216filed on Sep. 5, 2001.

BACKGROUND OF THE INVENTION

The invention relates to a process for the preparation of trialkylorthocarboxylates (orthoesters O) by the electrochemical oxidation ofalpha, beta-diketones or alpha, beta-hydroxyketones, the keto groupbeing present in the form of a ketal group derived from C₁- toC₄-alkylalcohols and the hydroxyl group optionally being present in theform of an ether group derived from C₁- to C₄-alkylalcohols (ketals K) ;in the presence of C₁- to C₄-alcohols (alcohols A), the molar ratio ofthe sum of the orthoesters (O) and the ketals (K) to the alcohols (A) inthe electrolyte being 0.2:1 to 5:1.

DESCRIPTION OF THE BACKGROUND

DE-A-3606472, for example, discloses non-electrochemical processes forthe preparation of trialkyl orthocarboxylates such as trimethylorthoformate (TMOF), chloroform being reacted with sodium methylate.

J. Org. Chem., 20 (1955) 1573, further discloses the preparation of TMOFfrom hydrocyanic acid and methanol.

J. Amer. Chem. Soc., (1975) 2546, J. Org. Chem., 61 (1996) 3256, andElectrochim. Acta, 42 (1997) 1933, disclose electrochemical processes bywhich C—C single bonds between C atoms each carrying an alkoxy group canbe oxidatively cleaved, but the specific formation of orthoester groupsis not described.

Russ. Chem. Bull., 48 (1999) 2093, discloses that vicinal diketonespresent in the form of their acetals are decomposed to the correspondingdimethyl dicarboxylates by anodic oxidation using high charge quantitiesand in the presence of a large excess of methanol (cf. p. 2097, column1, paragraph 5).

Canadian Journal of Chemistry, 50 (1972) 3424, describes the anodicoxidation of benzil tetramethyldiketal to trimethyl orthobenzoate in amore than 100-fold excess of methanol. According to the authors,however, the product yield is only 62% and the current efficiency 5%.

Journ. Am. Chem. Soc., (1963) 2525, describes the electrochemicaloxidation of orthoquinone tetramethylketal to the correspondingorthoester in a basic methanol solution. The reaction was carried out ina basic methanol solution with a substrate concentration of 10%. Theproduct yield was 77% with a current efficiency of 6% (16 F/mol). It hasnot been possible hitherto to prepare purely aliphatic orthoesterselectrochemically.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an electrochemicalprocess for the preparation of trialkyl orthocarboxylates in an economicmanner and especially with a high current efficiency, high productyields and a high selectivity.

We have found that this object is achieved by the process described atthe outset.

The process according to the invention is particularly suitable for thepreparation of orthoesters I of general formula I:

in which the radicals are defined as follows:

-   -   R¹ is hydrogen, C₁- to C₂₀-alkyl, C₂- to C₂₀-alkenyl, C₂- to        C₂₀-alkynyl, C₃- to C₁₂-cycloalkyl, C₄- to C₂₀-cycloalkylalkyl,        C₄- to C₁₀-aryl or optionally monosubstituted to trisubstituted        by C₁- to C₈-alkoxy or C₁- to C₈-alkoxycarbonyl;    -   R², R³ are C₁- to C₂₀-alkyl, C₃- to C₁₂-cycloalkyl or C₄- to        C₂₀-cycloalkylalkyl, or R² and R³ together form C₂- to        C₁₀-alkylene; and    -   R⁴ is C₁- to C₄-alkyl.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Said orthoesters are prepared starting from ketals II of general formulaII:

in which the radicals are defined as follows:

-   -   R⁵ and R¹⁰ are as defined for R¹;    -   R⁶ and R⁷ are as defined for R²;    -   R⁸ is hydrogen if R⁹ is as defined for R¹, or is as defined for        R²; and    -   R⁹ is as defined for R¹ or is —O—R².

It is also possible to obtain the orthoesters I in the form of a mixturewith ketals IV of general formula IV:

in which the radicals are defined as follows:

-   -   R¹¹ is as defined for R⁴;    -   R¹² is as defined for R²; and    -   R¹³ and R¹⁴ are as defined for R¹.

Said orthoesters are prepared starting from ketals II in which R⁹ isexclusively as defined for R¹.

The process according to the invention can be used to particularadvantage to prepare orthoesters of general formula Ia (orthoesters Ia):

in which the radicals are defined as follows:

-   -   R¹⁵ and R¹⁶ are as defined for R²;    -   R¹⁸ is as defined for R²;    -   R¹⁷ and R²⁰ are as defined for R⁴;    -   R¹⁹ is as defined for R²; and    -   X is C₂- to C₁₂-alkylene (orthoesters Ia).

Said orthoesters are prepared starting from ketals of general formulaIIa:

in which the radicals are defined as follows:

-   -   R²¹ and R²² are as defined for R²;    -   R²³ is as defined for R⁸;    -   R²⁴ is as defined for R⁹; and    -   Y is as defined for X (ketals IIa).

The ketals used according to the invention are obtainable by generallyknown preparative processes. In the case of ketals with functionalgroups, these are most easily prepared by starting from a precursorwhich carries a C—C double bond in place of the desired functionalgroup, and then functionalizing said double bond by standard methods(cf. Synthesis, (1981) 501–522).

The process according to the invention can also be used to particularadvantage to prepare orthoesters of formula Ib:

wherein

-   -   R¹ is hydrogen, C₁–C₂₀-alkyl, C₃–C₁₂-cycloalkyl or        C₄–C₂₀-cycloalkylalkyl;    -   R² and R³ are each C₁- to C₂₀-alkyl, C₃- to C₁₂-cycloalkyl or        C₄- to C₂₀-cycloalkylalkyl, or R² and R³ together form C₂- to        C₁₀-alkylene; and    -   R⁴ is C¹- to C⁴alkyl (orthoesters Ib),        starting from ketals II in which the radicals are defined as        follows:    -   R⁵ and R¹⁰ are as defined for group R¹ in orthoesters Ib; and    -   R⁶ to R⁹ are as defined for R² or R³ in orthoesters Ib (ketals        IIb).

Within the group of orthoesters Ib, the process according to theinvention can be used especially to prepare orthoesters of formula Ic:

-   -   wherein R¹ is hydrogen or C₁- to C₆-alkyl; and    -   R², R³ and R⁴ are methyl or ethyl (orthoesters Ic),        starting from ketals II in which the radicals are defined as        follows:    -   R⁵ and R¹⁰ are as defined for R¹ in orthoesters Ic; and    -   R⁶ to R⁹ are as defined for R² or R³ in orthoesters Ic (ketals        IIc).

In the ketals IIb and IIc the radicals R⁵ and R¹⁰ preferably have thesame definition.

The process according to the invention can be used to very particularadvantage to prepare methyl orthoformate (TMOF) or ethyl orthoformate ormethyl or ethyl orthoacetate (orthoesters Id), the correspondingstarting compounds being 1,1,2,2-tetramethoxyethane (TME) or1,1,2,2-tetraethoxyethane (ketals IId).

In the electrolyte the molar ratio of the sum of the orthoesters (O) andthe ketals K to the alcohols A is 0.2:1 to 5:1, preferably 0.2:1–2:1 andparticularly preferably 0.3:1 to 1:1.

The conducting salts present in the electrolysis solution are generallyalkali metal, tetra(C₁- to C₆-alkyl)ammonium or tri(C₁- toC₆-alkyl)benzylammonium salts. Suitable counterions are sulfate,hydrogensulfate, alkylsulfates, arylsulfates, halides, phosphates,carbonates, alkylphosphates, alkylcarbonates, nitrate, alcoholates,tetrafluoroborate or perchlorate.

The acids derived from the abovementioned anions are also suitable asconducting salts.

Methyltributylammonium methylsulfates (MTBS), methyltriethylammoniummethylsulfate or methyltripropylmethylammonium methylsulfates arepreferred.

Conventional cosolvents are optionally added to the electrolysissolution. These are the inert solvents with a high oxidation protentialwhich are generally conventional in organic chemistry. Dimethylcarbonate or propylene carbonate may be mentioned as examples.

The process according to the invention can be carried out in any of theconventional types of electrolysis cell. It is preferably carried outcontinuously with non-compartmentalized flow-through cells.

When the process is carried out continuously, the feed rate of theeducts is generally chosen so that the weight ratio of the ketals K usedto the orthoesters I formed in the electrolyte is 10:1 to 0.05:1.

The current densities used to carry out the process are generally 1 to1000 and preferably 10 to 100 mA/cm². The temperatures areconventionally −20 to 60° C. and preferably 0 to 60° C. The workingpressure is generally atmospheric pressure. Higher pressures arepreferably applied when the process is to be carried out at highertemperatures, in order to prevent the starting compounds or cosolventsfrom boiling.

Examples of suitable anode materials are noble metals such as platinum,or metal oxides such as ruthenium or chromium oxide or mixed oxides ofthe type RuO_(x)TiO_(x). Graphite or carbon electrodes are preferred.

Examples of suitable cathode materials are iron, steel, stainless steel,nickel, noble metals such as platinum, and graphite or carbon materials.Preferred systems have graphite as the anode and cathode or graphite asthe anode and nickel, stainless steel or steel as the cathode.

When the reaction has ended, the electrolysis solution is worked up bygeneral methods of separation. This is generally done by firstdistilling the electrolysis solution to give the individual compoundsseparately in the form of different fractions. These can be purifiedfurther, for example by crystallization, distillation or chromatography.

Experimental Section

EXAMPLE 1

A non-compartmentalized cell with graphite electrodes in a bipolararrangement was used. The total electrode surface area was 0.145 m²(anode and cathode). The electrolyte used was a solution consisting of 2mol of methanol to 1 mol of TME and containing 2% by weight of MTBS asthe conducting salt. Electrolysis was carried out at 300 A/m² and acharge quantity of 2 F, based on TME, was passed through the cell. Theelectrolysis temperature was 20° C. When the electrolysis had ended, theproducts were determined quantitatively by gas chromatography andqualitatively by GC coupled with MS. TMOF was formed with a selectivityof 77% for a TME conversion of 69%. The principal by-products weremethyl formate and methylal.

EXAMPLE 2

240.3 g of 1,1,2-trimethoxyethane, 320 g of methanol and 5.8 g ofammonium tetrafluoroborate were placed in an electrolysis cell with anelectrode surface area of 316.4 cm², but otherwise as described inExample 1, and subjected to electrolysis. The electrolysis conditionswere as described in Example 1. The electrolysis products contained 9.5GC area % of formaldehyde dimethylacetal and 5.9 GC area % of trimethylorthoformate.

EXAMPLE 3

89 g of 2,2,3,3-tetramethoxybutene (80% pure, prepared from diacetyl andtrimethyl orthoformate), 64 g of methanol and 1.7 g of ammoniumtetrafluoroborate were reacted in an electrolysis cell with an electrodesurface area of 298.8 cm², but otherwise as described in Example 1. Theelectrolysis conditions were as described in Example 1. The electrolysisproducts contained 1.7 GC area % of trimethyl orthoacetate for a currentquantity of 2 Faraday and 18 GC area % for a current quantity of 8 F.

EXAMPLE 4

In an electrolysis operated continuously at a current density of 310A/m² on graphite electrodes with amethanol-to-1,1,2,2-tetramethoxyethane feed of 1.5 mol to 1 mol and anMTBS content of 8% by weight, the electrolysis products contained TMOFwith a selectivity of 95% and a current efficiency of 78% for a TMEconversion of 41%.

1. A process for the preparation of a trialkyl orthocarboxylate(orthoester O), comprising: electrochemically oxidizing analpha,beta-diketone or an alpha, beta-hydroxyketone, wherein the ketogroups of the alpha, beta-diketone compound or the keto group of thealpha, beta-hydroxyketone compound are present in the form of ketalgroups derived from C₁- to C₄-alkylalcohols and the hydroxyl group ofthe alpha-beta-hydroxyketone optionally being present in the form of anether group derived from C₁- to C₄-alkylalcohols (ketals K), in thepresence of a C₁- to C₄-alcohol (alcohols A) medium that contains anmedium electrolyte, the molar ratio of the sum of the orthoester O andthe ketals K to the alcohols A in the medium ranging from 0.2:1 to 5:1.2. The process as claimed in claim 1, wherein the electrolyte is aconducting salt of a tetra(C₁- to C₆-alkyl)ammonium or a tri(C₁- toC₆alkyl)benzylammonium cation with sulfate, hydrogensulfate,alkylsulfates, arylsulfates, halides, phosphates, carbonates,alkylphosphates, alkylcarbonates, nitrate, alcoholates,tetrafluoroborate or perchlorate as a counterion.
 3. The process asclaimed in claim 1, wherein the electrolyte is a conducting salt whichis methyltributylammonium ethylsulfate, methyltripropylammoniummethylsulfate, methyltriethylammonium methylsulfate ortetramethylammonium methylsulfate.
 4. The process as claimed in claim 1,which is conducted in a non-compartmentalized electrolysis cell.
 5. Theprocess as claimed in claim 1, wherein the charge quantity per mol ofoxidized alpha, beta-diketone or alpha, beta-hydroxyketone is 2 to 4 F.6. The process as claimed in claim 1, wherein the electrochemicaloxidation is conducted at a current density of 1 to 1000 mA/cm².
 7. Theprocess as claimed in claim 1, wherein the electrochemical oxidation isconducted at a current density of 10 to 100 mA/cm².
 8. The process asclaimed in claim 1, wherein the electrochemical oxidation is conductedat a temperature ranging from −20 to 60° C.
 9. The process as claimed inclaim 8, wherein the electrochemical oxidation is conducted at atemperature ranging from 0 to 60° C.
 10. A process for the preparationof a trialkyl orthocarboxylate, comprising: electrochemically oxidizinga ketal II of formula II:

wherein the radicals are defined as follows: R⁵ and R¹⁰ are as definedfor R¹ below; R⁶ and R⁷ are as defined for R² below; R⁸ is hydrogen ifR⁹ is as defined for R¹, or is as defined for R²; and R⁹ is as definedfor R¹ or is —O—R², to an orthoester I that is a compound of formula I:

wherein the radicals are defined as follows: R¹ is hydrogen, C₁- toC₂₀-alkyl, C₂- to C₂₀-alkenyl, C₂- to C₂₀-alkynyl, C₃- toC₁₂-cycloalkyl, C₄- to C₂₀-cycloalkylalkyl, C₄- to C₁₀-aryl oroptionally monosubstituted to trisubstituted by C₁- to C₈-alkoxy or C₁-to C₈-alkoxycarbonyl; R² and R³ are each C₁- to C₂₀-alkyl, C₃- toC₁₂-cycloalkyl or C₄- to C₂₀-cycloalkylalkyl, or R² and R³ together forma C₂- to C₁₀-alkylene; and R⁴ is C₁- to C₄-alkyl.
 11. The process asclaimed in claim 10, wherein the orthoester I is an orthoester compoundof the formula Ic:

wherein: R¹ is hydrogen or C₁ to C₆-alkyl and R², R³ and R⁴ are methylor ethyl, and the ketal II has formula II:

wherein the radicals are defined as follows: R⁵ and R¹⁰ have the meaningof R¹ and R⁶ to R⁹ have the meaning of R² or R¹.
 12. The process asclaimed in claim 11, wherein the orthoester I is methyl or ethylorthoformate or methyl or ethyl orthoacetate, and the ketal II is1,1,2,2-tetramethoxyethane or 1,1,2,2-tetraethoxyethane, or1,1,2,2-tetramethoxypropane or 1,1,2,2-tetraethoxypropane, or2,2,3,3tetramethoxybutane or 2,2,3,3tetraethoxybutane.
 13. A process forthe preparation of a trialkyl orthocarboxylate, comprising:electrochemically oxidizing a ketal II of formula II:

wherein the radicals are defined as follows: R⁵ and R¹⁰ are eachhydrogen, C₁- to C₂₀-alkyl, C₂- to C₂₀-alkenyl, C₂- to C₂₀-alkynyl, C₃-to C₁₂-cycloalkyl, C₄- to C₂₀-cycloalkylalkyl, C₄- to C₁₀-aryl oroptionally monosubstituted to trisubstituted by C₁- to C₈-alkoxy or C₁-to C₈-alkoxycarbonyl; R⁶ and R⁷ are each C₁- to C₂₀-alkyl, C₃- toC₁₂-cycloalkyl or C₄- to C₂₀-cycloalkylalkyl, or R⁶ and R⁷ together forma C₂- to C₁₀-alkylene; R⁸ is hydrogen if R⁹ is as defined for R⁵ andR¹⁰, or is as defined for R⁶ and R⁷, and R⁹ is hydrogen, C₁- toC₂₀-alkyl, C₂- to C₂₀-alkenyl, C₂- to C₂₀-alkynyl, C₃- toC₁₂-cycloalkyl, C₄- to C₂₀-cycloalkylalkyl, C₄- to C₁₀-aryl oroptionally monosubstituted to trisubstituted by C₁- to C₈-alkoxy or C₁-to C₈-alkoxycarbonyl, to a mixture of an orthoester I of formula I:

wherein the radicals are defined as follows: R¹ is hydrogen, C₁- toC₂₀-alkyl, C₂- to C₂₀-alkenyl, C₂- to C₂₀-alkynyl, C₃- toC₁₂-cycloalkyl, C₄- to C₂₀-cycloalkylalkyl, C₄- to C₁₀-aryl oroptionally monosubstituted to trisubstituted by C₁- to C₈-alkoxy or C₁-to C₈-alkoxycarbonyl; R² and R³ are each C₁- to C₂₀-alkyl, C₃- toC₁₂-cycloalkyl or C₄- to C₂₀-cycloalkylalkyl, or R² and R³ together forma C₂- to C₁₀-alkylene; and R₄ is C₁- to C₄-alkyl and a ketal IV offormula IV

wherein the radicals are defined as follows: R¹¹ is C₁- to C₄-alkyl; R¹²is C₁- to C₂₀-alkyl, C₃- to C₁₂-cycloalkyl or C₄- toC₂₀-cycloalkylalkyl; and R₁₃ and R¹⁴ is hydrogen, C₁- to C₂₀-alkyl, C₂-to C₂₀-alkenyl, C₂- to C₂₀-alkynyl, C₃- to C₁₂-cycloalkyl, C₄- toC₂₀-cycloalkylalkyl, C₄- to C₁₀-aryl or optionally monosubstituted totrisubstituted by C₁- to C₈-alkoxy or C₁- to C₈-alkoxycarbonyl.
 14. Aprocess for the preparation of a trialkyl orthocarboxylate, comprising:electrochemically oxidizing a ketal II of formula IIa:

wherein the radicals are defined as follows: R¹² and R²² are each C₁- toC₂₀-alkyl, C₃- to C₁₂-cycloalkyl or C₄- to C₂₀-cycloalkylalkyl; R²³ ishydrogen; R²⁴ is hydrogen, C₁- to C₂₀-alkyl, C₂- to C₂₀-alkenyl, C₂- toC₂₀-alkynyl, C₃- to C₁₂-cycloalkyl, C₄- to C₂₀-cycloalkylalkyl, C₄- toC₁₀-aryl or optionally monosubstituted to trisubstituted by C₁- toC₈-alkoxy or C₁- to C₈-alkoxycarbonyl; and Y is as defined for X below,to an orthoester (Ia) that is a compound of formula la:

in which the radicals are defined as follows: R¹⁵ and R¹⁶ are as definedfor R²¹ and R²²; R¹⁸ is as defined for R²¹ and R²²; R¹⁷ and R²⁰ are C₁-to C₄-alkyl; R¹⁹ is as defined for R²¹ and R²²; and X is C₂- toC₁₂-alkylene.